Helium is mainly produced by the radioactive decay of heavier elements such as uranium and thorium. The helium formed upon radioactive decay typically percolates slowly through rock into large cavities that also contain reserves of natural gas. Helium diffusion from these pockets results in relatively low atmospheric concentration (e.g., 5 ppmv). Once released from the soil or water, it is irreversibly lost in space. Consequently, helium is considered a nonrenewable resource.
In the past, pressure swing adsorption systems have been used to separate or remove helium from helium-containing feed gases. Such pressure swing adsorption (PSA) or vacuum pressure swing adsorption (VPSA) systems have had low helium purity and per pass helium recovery when using a single stage PSA or VPSA process alone to recover helium. Prior art efforts to achieve enhanced helium purity and recovery have included utilizing a combination of PSA systems and membranes, or PSA and cryogenic systems, or serial arrangements of PSA or VPSA processes. In some prior art two stage VPSA systems, the same or different number of beds are used in the stages with PSA or VPSA cycles utilizing rinse steps and additional compression equipment(s). Such systems and processes can result in higher capital and/or operating costs.
Helium-containing feed gas can contain several components, e.g. a feed stream may contain large concentrations of N2 and trace quantities of hydrocarbons and/or other contaminants. Prior art systems that use primarily all carbon beds to remove the contaminants in the helium-containing feed gas result in an inefficient, uneconomical or unsustainable helium recovery system due to the degradation of carbon adsorbent and excessive use of compression equipment that are necessary for various steps in the helium recovery process. Given that the helium-containing feed gas composition can vary widely, significant challenges have existed to determine efficient adsorbent selection for a combination of adsorbates, adsorbent configuration in the adsorber, and purity control to achieve an efficient He recovery from PSA/VPSA processes.
U.S. Pat. Nos. 5,089,048 and 5,080,694 to Knoblauch et al. disclose PSA processes for extracting helium from a relatively helium poor gas mixture, e.g., natural gas containing 2-10% helium by volume.
U.S. Pat. No. 5,542,966 to D'Amico et al. relates to pressure swing adsorption processes to recover helium from source streams of less than about 10 percent by volume helium and concentrate the helium to a concentration of greater than about 98 percent by volume. Two stages of pressure swing adsorption are used in series. The source of the helium gas can be natural gas wells. The source gas can contain hydrocarbons, although in many instances, the primary gas other than helium will be nitrogen.
U.S. Pat. No. 5,707,425 D'Amico et al. is directed to the recovery of helium gas from gas streams containing about 25% by volume or more of helium. Two PSA processes are used in a serial arrangement.
U.S. Pat. No. 5,632,803 to Stoner et al. relates to a hybrid membrane/PSA process for producing helium product streams at purities in excess of 98% from feed stock containing from 0.5 to 5.0% helium. The membrane is placed upstream of two PSA processes, and all of the separation units are arranged in a serial configuration. According to this invention, the hybrid system utilizes at least one stage of membrane enrichment followed by at least two stages of pressure swing adsorption.
U.S. Pat. No. 5,224,350 to Mehra relates to a system of taking a subquality nitrogen-rich natural gas stream containing more than 0.1 mol % helium to produce a rich solvent bottoms stream that is flashed twice to produce a methane-rich gas product and a nitrogen-helium product which is fed to at least one membrane unit. A reject nitrogen stream and a crude helium stream are discharged from this unit. The crude helium stream is either compressed to a pressure within the range of 200 to 3,000 psia or is compressed to no more than 1,000 psia and fed to a PSA unit which produces a reject nitrogen stream and a purified helium stream which is compressed to a pressure within the range of 200 to 3,000 psia.
U.S. Pat. No. 6,630,011 B1 to Baker al. relates to a process for treating natural gas or other methane-rich gas to remove excess nitrogen. The process relies on two-stage membrane separation, using methane-selective membranes for the first stage and nitrogen-selective membranes for the second stage.
U.S. Pat. No. 6,179,900 B1 to Behling et al. describes processes for the separation/recovery of gases where the desired component to be separated from the mixture is present in low molar concentrations and/or low to moderate pressures. A combined membrane/PSA process is utilized for the separation/recovery of gaseous components which are present in the stream at low pressures and/or molar contents. The membrane unit is positioned at the upstream end of the PSA process.
U.S. Pat. No. 7,294,172 B2 to Baksh et al. discloses a highly efficient and low cost noble gas recovery system to recover and conserve valuable gas (e.g., helium) from various applications (e.g., atomization furnaces, plasma furnaces, sputtering, etc.). In addition, U.S. Pat. No. 7,169,210 B2 to Baksh et al. relates to a control system for a helium recovery system.
There remains a need for improved methods and systems for helium recovery from low helium-containing feed gases (i.e. containing less than about 10% helium by volume and more typically, less than about 5% helium by volume) at high (e.g., >90%) helium recoveries, high throughput, and improved capital and operating costs.